The simplified Bishop method assumes zero Xi forces between blocks. The method is based on satisfying the moment equation of equilibrium and the vertical force equation of equilibrium.
The factor of safety SF is found through a successive iteration of the following expression:
pore pressure within the block
effective values of the soil parameters
inclination of the segment of the slip surface
horizontal width of the block
Bishop, A.W. (1955) "The Use of the Slip Circle in the Stability Analysis of Slopes", Geotechnique, Great Britain, Vol. 5, No. 1, Mar., pp. 7-17.